water column

Short definition from Wiktionary

Noun

1. 1
   A notional column of water from the surface to the bottom in a natural setting, notable for the differences in physical and chemical properties at various depths.
2. 2
   The zone in which ascent and descent occur.
3. 3
   A column alongside a railway track used to replenish a steam engine's water supply.
4. 4
   The column of water inside the graduated cylinder of a pressure gauge that measures pressure in inches of water.